docs: expose quick start, security, and social preview

[bobleer]

Summary

• replace the top-level Docs directory link with the existing first-run quick start
• expose the existing security policy beside the download link
• keep the English and Chinese visitor paths equivalent
• add a 1280×640, sub-1 MB GitHub Social Preview candidate without license or benchmark claims

Why

docs/ has no landing page, so the current top link sends new visitors into an internal file listing. The first-run and security pages already exist and are more useful immediately after a social referral.

GitHub currently renders the default owner-avatar social card. The existing png/og-image.png includes broad open-source/MIT and point-in-time benchmark claims, so this PR adds a separate conservative candidate instead of replacing it. A repository admin can upload png/github_social_preview.png under Settings → General → Social preview after review.

Verification

• git diff --check
• bilingual target/file assertions
• GitHub Markdown rendering check
• pnpm run check:repo-hygiene
• preview is exactly 1280×640 and 564,432 bytes
• OCR/visual review confirms it contains no MIT, open-source, Office, or benchmark copy

AI assistance

AI-assisted. Testing level: fully tested for this documentation/asset scope. The preview background was generated with ImageGen; the BitFun wordmark, repository UI crop, exact text, dimensions, and final composition were applied deterministically from repository assets.

Admin-only follow-up (not changed by this PR)

• upload png/github_social_preview.png as the repository Social Preview
• consider shortening About to: Desktop AI agent for code and real-world work, powered by a Rust runtime, task-specific Mini Apps, and self-hosted multi-device control.
• consider adding the discovery topics ai-agent, coding-agent, desktop-app, self-hosted, rust, tauri, and llm

[bobleer]

@GCWing The repository-level changes are now merged. One admin-only follow-up remains: upload png/github_social_preview.png under Settings → General → Social preview. The current openGraphImageUrl is still GitHub’s default card. The PR body also lists the conservative About sentence and seven discovery topics; I cannot apply those with the current Write-only permission.